1. Field of the Invention
The invention relates in general to a method for determining a compensation value for tracking error signal, and more particularly to a method for determining a compensation value for tracking error signal before generating a tracking error signal by differential phase detection (DPD).
2. Description of the Related Art
Nowadays technology is developing very soon, and discs of high storage capacity have become an essential medium carrier for people's daily lives due to widespread application of pictures, music, movies and computer software. The discs, including CD and DVD, are very popular to people due to features of high capacity, small volume and high data security. For this reason, the optical disk drive capable of reading a CD and DVD becomes essential equipment for a personal computer.
In the process of playing a DVD on the conventional optical disc drive, first, place the DVD in the optical disc drive, and rotate it by a spindle motor of the optical disc drive. Next, move an optical pick-up head of the optical disk drive to one side of the rotating DVD. Then, finely adjust a focus servo system of the optical disc drive to perform a focusing operation. Afterwards, finely adjust a tracking servo system of the optical disc drive and generate a tracking error signal by DPD. It should be noted that a compensation value is determined by the optical disc drive to compensate for the practical tracking error signal before the tracking error signal is generated. According to the generated tracking error signal, it can be determined whether the optical disc drive performs a correct tracking operation. As soon as the correct tracking is done, the optical disc drive can reproduce data on DVD.
Referring to FIG. 1A, a partial schematic diagram of a conventional optical disc drive for reading a DVD is shown. The DVD 17 has a first reflection layer 19 and a second reflection layer 20. The optical disc drive includes at least a spindle motor 15 and an optical pick-up head 16. The turntable 15a of the spindle motor 15 is for engaging with the positioning hole 18 at the center of the DVD 17 and the spindle motor 15 can rotate the DVD 17 at a low speed, corresponding to an 8× access speed of the optical disc drive for instance. Moreover, the optical pick-up head 16 has to focus and keep the laser in a track of the first reflection layer 19 or the second reflection layer 20 so as to read data on DVD 17 as the DVD 17 rotating. As shown in FIG. 1C, the first reflection layer 19 and the second reflection layer 20 have respectively a few circles of first tracks and second tracks, such as the first tracks 19a˜19f, and the second tracks 20a˜20f. The first track 19a and the second track 20a are respectively the innermost tracks of the first reflection layer 19 and the second reflection layer nearest to the positioning hole 18 while the first track 19f and the second track 20f respectively the outermost tracks of the first reflection layer 19 and the second reflection layer farthest to the positioning hole 18.
Referring to FIG. 1B, a flowchart of the conventional method for determining the compensation value for tracking error signal is shown. First, in step 11, respectively read the data on the first track 19a and the second track 20a to perform tracking operation when the optical disc drive rotates the DVD 17 at a low speed. Next, in step 12, respectively generate a first compensation value for tracking error signal and a second compensation value for tracking error signal, such as the values s and v as shown in FIGS. 1D and 1E, by using the optical disc drive. Afterwards, in step 13, respectively use the first compensation value and the second compensation value to be the compensation values for tracking error signal corresponding to the first tracks 19b˜19f and the second tracks 20b˜20f as the DVD 17 rotating. That is, the first compensation value for tracking error signal is always the value s no matter how fast the disc is rotated while the second compensation value for tracking error signal always the value v no matter how fast the disc is rotated.
However, when the optical disc drive rotates the DVD 17 at a higher speed, corresponding to a 12× data access speed for instance, due to the electrical features of the optical disc drive, the farther the first track departs from the positioning hole 18, the larger the corresponding first compensation value s for tracking error signal becomes. Similarly, the farther the second track departs from the positioning hole 18, the larger the corresponding second compensation value v for tracking error signal becomes. As shown in FIGS. 1D and 1E, the compensation values for tracking error signal corresponding to the first tracks 19a˜19f are respectively T1˜T6. The value T1 is substantially equal to the value s, but the values T2˜T6 are all larger than s. In the same way, the compensation values for tracking error signal corresponding to the second tracks 20a˜20f are respectively R1˜R6. The value R1 is substantially equal to the value v, but the values R2˜R6 are all larger than v. Consequently, the optical disc drive will generate an incorrect tracking error signal according to an incorrect compensation value for tracking error signal, thereby reducing its tracking effect.